On VLSI and ULSI semiconductor chips, Al and alloys of Al are used for conventional chip interconnect/wiring material. However, more recently copper and alloys of copper have been developed as chip interconnect material. The use of copper and copper alloys results in improved device performance when compared to Al and its alloys.
In the fabrication of semiconductor devices, the metallic interconnect material or wiring such as the copper or its alloys is typically planarized after deposition.
Polishing slurries used for this planarization are typically aqueous suspensions comprised of a metal oxide abrasive (such as alumina), organic acids, surfactants, and a suitable oxidizing agent. This process is known as chemical-mechanical polishing (CMP). The oxidizing agent works to enhance mechanical removal of material via a corrosion assisted process. Such oxidizing agents employed in commercially-available or proprietary slurries are typically inorganic metal salts such as FeNO3, or KIO3, and also hydrogen peroxide, present in significant concentrations. Other chemicals added to slurries to improve dispersion or otherwise enhance performance often are organic acids (e.g. citric acid). Sodium, potassium, and iron salts and/or compounds are frequently used in slurry formulations, and significant amounts of these metal ion impurities can remain on the wafer after polishing and post-polish cleaning.
Therefore, a tendency exists for various particulate contaminants to remain on the polished surface. The particulate materials are extremely difficult to remove. This is particularly problematic since the removal must not adversely affect the polished surface.
Furthermore, since the polishing slurries typically contain an oxidizing agent, an oxide layer usually is present on the copper due to oxidization of the copper during the CMP process. This layer may adversely affect the electrical characteristics of the device, and is preferably removed. In fact, this layer may also contribute to the contamination.
Accordingly, a need exists for a post chemical mechanical polishing cleaning chemistry that removes metallic and particulate contamination. In addition, it is desired that the cleaning step remove any residual copper oxides and/or other non-desirable surface films, leaving a bare copper surface.
The problems of developing such a cleaning is further exacerbated by the need to minimize etching of the copper as well as avoiding increased surface roughness to any significant extent.
Moreover, compositions containing fluorides have been suggested. However, it has become desirable to provide fluoride-free compositions due to environmental considerations.